Metamaterials are materials with artificial electromagnetic properties defined by the sub-wavelength physical structure of the materials, rather than their chemical composition. Metasurfaces are a category of metamaterials that comprise a two-dimensional pattern of repeating structures, having dimensions (pitch and thickness) less than the target wavelength of the radiation with which the metasurface is designed to interact.
Double-negative (DNG) metamaterials have attracted much research interest in recent years, although practical applications of such metamaterials are yet to develop. A DNG metamaterial has both negative electrical permittivity (ε) and negative magnetic permeability (μ) in a wavelength range of interest. Such metamaterials exhibit negative refractive index (n), resulting in negative refraction and backward phase propagation. Designs of DNG metamaterials for use in the optical range have been described, for example, by Burgos et al., in “A single-layer wide-angle negative-index metamaterial at visible frequencies,” Nature Materials 9 (May, 2010), pages 407-412; and by Valentine et al., in “Three-dimensional optical metamaterial with a negative refractive index,” Nature 455 (Sep. 18, 2008), pages 376-380. Both of these publications are incorporated herein by reference. The terms “optical” and “light,” as used in the context of the present patent application and in the claims, refer to electromagnetic radiation in any of the visible, infrared and ultraviolet ranges.
Lenses made from conventional materials, having a positive index of refraction, are limited in their resolution by the well-known Abbe diffraction limit, and thus cannot resolve structures of dimensions less than about half the imaging wavelength. In other words, the finest dimensions that can be resolved using visible light imaging, for example, are on the order of 0.25 μm. On the other hand, researchers have recognized that the properties of DNG metamaterials can be used, at least in theory, to produce perfect lenses, i.e., slab lenses that create an image of an object with essentially unlimited resolution. The resolution of the DNG lens may be reduced substantially, however, if the slab material is lossy (i.e., if it absorbs or scatters the radiation that it is intended to focus), as explained, for example, by Collin in “Frequency dispersion limits resolution in Veselago lens,” Progress in Electromagnetics Research B 19 (2010), pages 233-261; and by Smith, et al., in “Limitations on subdiffraction imaging with a negative refractive index slab,” Applied Physics Letters 82 (2003).
Optical devices based on metamaterials have been described in the patent literature. For example, U.S. Pat. No. 8,094,378, to Kildishev et al., describes a design method for structures for controlling the flow of electromagnetic energy at a sub-wavelength scale. Plane waves incident at a first surface of a planar lens of this type are said to be focused to a spot size substantially smaller than a wavelength, so as to interact with objects at the focal point, or be re-radiated. As another example, U.S. Pat. No. 8,599,489, to Shalaev et al., describes a “tunable super-lens” (TSL) for nanoscale optical sensing and imaging of bio-molecules and nano-manufacturing. The TSL utilizes negative-index materials that operate in the visible or near-infrared range. Additional patents in this field include EP2269110, WO2010144170, and U.S. Pat. No. 8,180,213.